It is becoming common place to implement mobile devices with multiple modes to enhance roaming capabilities. Each mode employs a different radio access technology (RAT) which is associated with a Radio Access Network (RAN) and a core network. Examples of modes that may be implemented in a multi-mode device include cdma2000 (commonly referred to as CDMA 1x and 1xEV-DO), GSM/GPRS/EDGE, and UMTS/HSPA. In addition, a multimode device may support other air interfaces such as IEEE 802.11a/b/g/n, 802.12.16/16e and similar standards.
The cdma2000 air-interface currently uses the 850 MHz band (also known as cellular band or band class 0) and the 1900 MHz band (also known as the PCS band or band class 1) in North America. The GSM/GPRS/EDGE air interface typically uses the 900 MHz band (also known as E-GSM band) and the 1800 MHz band (also known as DCS band) in Europe, 900 MHz band in Asia and Africa, and uses the 850 MHz band and the 1900 MHz band in North America and the Caribbean. UMTS/HSPA currently uses the 2100 MHz band (also known as IMT-2000 band) in Europe and Asia, and uses the 850 MHz band and the 1900 MHz band in North America. In addition, the 900 MHz band is currently open for UMTS/HSPA use in several countries and the 1800 MHz band will be open for UMTS/HSPA use in near future.
Each of the frequency bands has an associated uplink band and downlink band. Each uplink band and each downlink band is typically divided up into 5 MHz blocks. In addition, each uplink block is paired with a downlink block. Typically the regulatory board in a given country assigns one or multiple blocks of a frequency band to a network operator or carrier. An assigned frequency block within a frequency band is used by an assigned carrier using a particular radio access technology. Some frequency bands can be used by multiple radio access technologies. For example, 850 MHz and 1900 MHz bands are used by network operators who have deployed cdma2000, GSM/GPRS/EDGE, and UMTS/HSPA in these bands. In this case, each radio access technology uses non-overlapping frequency blocks within the same band with an appropriate guard band in-between. Similarly, 900 MHz and 1800 MHz bands, which are currently used by GSM/GPRS/EDGE, may also be available for UMTS/HSPA radio access technologies in the future based on recommendation of regulatory boards. A network operator may support both the GSM/GPRS/EDGE and the UMTS/HSPA radio access technologies since these air interfaces can share the same core network although the radio access networks are different. However, a cdma2000 network operator typically will not support GSM or UMTS radio access technology since both the radio access network and the core network are different between these air interfaces.
In some implementations of network scanning by a mobile device, the GSM and UMTS bands are further organized into band groups to take advantage of usage or typical deployment of these bands based on geographical location. For example, band group 1 may include GSM 850 MHz and 1900 MHz, and UMTS 850 MHz and 1900 MHz, this band group comprising frequency bands used in North America. Band group 2 may include GSM 900 MHz and 1800 MHz, and UMTS 900 MHz, 1800 MHz and 2100 MHz, this band group comprising frequency bands that are used in the rest of the world. As a specific example, the 1900 MHz band includes an uplink band in the range 1850 MHz to 1910 MHz, and a downlink band in the range 1930 MHz to 1990 MHz. A specific example of assignment of the 5 MHz blocks in a particular location might include the following for the downlink:
1930 MHz-1935 MHz: CDMA—network operator 1
1935 MHz-1940 MHz: GSM—network operator 2
1940 MHz-1950 MHz: UMTS—network operator 2
1950 MHz-1960 MHz: CDMA—network operator 3
1960 MHz-1965 MHz: GSM—network operator 4
When a multimode mobile device initiates network scanning for available service upon power-up or loss of network connectivity, it typically attempts to find a network using the same radio access technology that was last used. The frequency scans for different radio access technologies are separate even when scanning is performed on the same frequency band. In other words, when the mobile device scans for available networks of a given radio access technology, the underlying scanning mechanism is tightly coupled with the physical layer characteristics of the associated radio access technology and varies from one radio access technology to another. For example, the scanning mechanisms for CDMA, GSM, and UMTS in the same 1900 MHz band are distinctly different and the actual scanning is done separately for each radio access technology in case of a single receiver implementation. In addition, the mobile device initially selects the same frequency band or band group where it last acquired service. For example, if a mobile device was previously operating using GSM on the 1900 MHz band prior to power off, the mobile device after subsequent power on will initiate a GSM network scan on 1900 MHz. This will be band group 1 for the example above where the mobile device was previously using 1900 MHz GSM. After that, in typical implementations the multi-mode mobile device switches to performing scanning for GSM or UMTS using band group 2, before switching to performing scanning for a CDMA network.
Network scanning may, for example, involve looking at all the relevant frequencies in a frequency band. For example, a GSM scan in the 1900 MHz band may involve scanning a total of 299 200-KHz-wide frequency channels. Excessive scanning can be costly for the mobile device in terms of battery life as well slowness in acquiring service.